In my own personal experience I've found them to be handy when I'm following my fellow artists who do a wide variety of events, like solo shows, book signings, table shows, etc. in addition to their comic book announcements. In that respect I like being on the mailing list to see an aggregation of all their events and locations each month or season.
I don't necessarily think it would be handy for someone who didn't need to make a lot of announcements in one place about where to come physically see them over the next couple of months, or who didn't have a bunch of different articles on different sites to point people to, or etc. I think it's most handy (at least to me) as a master posting of all the stuff you've got going on in a given interim.
I also think it's good to have the interims be relatively far spaced apart, if you decide to do a list. Once every couple of months maybe, or once a quarter.